In a typical treatment operation a gas stream is passed upwards through a treatment column, such as a scrubbing tower, and a liquid stream passes downwards through the column countercurrent to the gas. One or both of the fluid feed streams may contain contaminants, which may be gaseous, liquid or solid. On contact between the fluid streams there occur mass and energy transfers at a rate affected by a number of factors including the respective stream temperatures and the extent of contact. The transfers can be improved by the presence of packing elements in the column and this is especially the case if the gas is passed upwards at a velocity sufficient to fluidise the packing elements. The so-formed fluidised bed of packing elements represents turbulent conditions which increase the contact between the fluids and therefore further increase the mass and energy transfer.
Such fluidised bed operations include one or more of absorption, desorption, distillation, heat transfer for heating or cooling, scrubbing, stripping, and particulate or droplet transfer.
Improvements in transfer rates in fluidised beds are much sought after. The aim is to improve energy efficiency of the bed, and thus to increase the performance of a given column and bed or to maintain a given performance while reducing column size and/or bed volume.
Historically the packing elements were typically in the form of hollow spheres. It has been however been shown that non-spherical shapes may be more energy-efficient. Compared with spherical elements these alternative shapes provide an enhanced tumbling action which increases the turbulence imparted to the fluid streams and thus improves the contact between them. German patent specification No. 3613151-A shows improvements in scrubbing efficiencies by using ellipsoidal elements. PCT patent specification WO91/08048 describes elements with their centre of gravity offset from the centre of symmetry (COG/COS offset) to enhance their tumbling motion. U.S. Pat. No. 5,588,986 further teaches that using COG/COS offset, varying the size, shape and density of the element, and controlling the velocities of the gas and liquid streams permits control of the tumble velocity and thus of the transfer rate.
The tumble velocity can be determined by the pressure gradient (quantified as pressure drop ΔP over the settled or pre-fluidised bed height Ho) across elements in the fluidised bed, whereby the higher the pressure gradient across the elements, the higher the tumble velocity for a given element. There is however a limit to the tumble velocity that can be achieved by varying the density and COG/COS offset.
Increases to the pressure gradient by different means, for example by increasing element density alone, without an increase in tumble velocity can lead to a loss in energy efficiency characterised by high pressure drop (ΔP) per number of transfer units (NTU).